This work has generated a series of novel and known cyclopentenyl (CPE) nucleoside isosteres. The target compounds synthesized comprise: CPE-adenosine (neplanocin A, 4), CPE-3-deazaadenine (3-deazaneplanocin A, 5), CPE-cytosine (11), CPE-uracil (12), CPE-8-azaadenine (8-azaneplanocin A, 6), 2', 3'-dideoxy-CPE-cytosine (14), and 2',3'-dideoxy-CPE-adenine (10a). Most of these compounds are endowed with unique antitumor or antiviral activity that appears to be related to the unsaturaton present in the carbocycle portion of the molecule. Efforts will continue to exploit this area to its fullest extent using natural and altered aglycone bases. Several new methods of preparation for these compounds have been developed and other alternative syntheses are being explored.